1. Field of the Invention
The present invention relates to a digital communication system. More particularly, it relates to a digital communication system employed in mobile radio communication which is immune from multipath interference.
2. Description of the Prior Art
The inventors proposed "A MODULATION TECHNIQUE SUFFERING FROM LESS TIMING FLUCTUATION OF EYE PATTERNS IN MULTIPATH FADING", in a technical meeting sponsored by the Institute of Electronics and Communication Engineers in Japan held on Aug. 30, 1984. This article describes a DSK (Double Phase Shift Keying) of a type in which a phase is shifted every half time slot by .pi./2. However, an amount of phase-shift is not limited to .pi./2, but any amount .DELTA..theta./2 may be taken, which is referred to as ".DELTA..theta./2-DSK".
FIG. 1 is a diagram for explaining a .DELTA..theta./2-DSK signal, FIG. 2 is a diagram showing a T/2 differential detector, FIG. 3 is a diagram showing the relation between a D wave and a U wave, and FIG. 4 is a diagram showing demodulation outputs in sections b and d and a section c in the .DELTA..theta./2-DSK signal.
The .DELTA..theta./2-DSK signal shown in FIG. 1 is such that the phase of a carrier wave is shifted by +.DELTA..theta./2 during a period of T/2 in the first half of a time slot and further by .DELTA..theta./2 during a period of T/2 in the second half thereof clockwise or counterclockwise, in correspondence to a digital binary information symbol. In order to demodulate an original binary information symbol from the .DELTA..theta./2-DSK signal shown in FIG. 1, a T/2 differential detector 3 shown in FIG. 2 is employed. In FIG. 2, the .DELTA..theta./2-DSK signal is applied to an input terminal 1, and also to a delay circuit 3a having a delay length of T/2. The .DELTA..theta./2-DSK signal is multiplied by its replica delayed by T/2 by a multiplier 2. An output of the multiplier 2 is taken out through a low pass filter 4 to an output terminal 5.
Referring now to FIGS. 1 and 2, description is made on operation of a demodulator. The .DELTA..theta./2-DSK signal is such that the phase of the carrier wave is shifted by +.DELTA..theta./2 during a period of T/2 in the first half time slot and further by +.DELTA..theta./2 during a period of T/2 in the second half time slot for a binary information symbol "1", while the phase of the carrier wave is shifted by -.DELTA..theta./2 during a period of T/2 in the first half time slot and further by -.DELTA..theta./2 during a period of T/2 in the second half time slot for a binary information symbol "0".
Description is now made on operation of this system in multipath propagation which causes a problem in mobile radio. Assuming that .tau. is an arrival time difference between the D wave and the U wave which are the .DELTA..theta./2-DSK signals having the identical content and received through two routes having different propagation path lengths, the relation is shown in FIG. 3. Even when the D wave and the U wave are out-of-phase and cancel with each other in sections b and d, the phase difference between th D and U waves in the section c is .DELTA..theta./2, thus the resultant of the D and U waves is not canceled.
When a combined wave of the D wave and the U wave having such a relation is demodulated in the T/2 differential detector shown in FIG. 2, a demodulation output e(t) is represented by the following expressions: ##EQU1## where cos .DELTA..theta./2 is referred to as the amplitude coefficient, .rho. represents the relative amplitude ratio of the U wave to the D wave and .phi. represents the carrier phase difference between the D wave and the U wave. The relation between the outputs in sections b, d and in section c is illustrated in FIG. 4.
As seen from FIG. 4, in the .DELTA..theta./2-DSK signal, the demodulation outputs in the sections b and d and in the section c are in compensating relation with respect to the change of the carrier phase difference .phi., such that one is decreased while the other is increased.
More specifically, in a simple BPSK (Binary Phase Shift Keying) signal, when the phase difference .phi. between the D wave and the U wave approaches to .pi. (rad), the demodulation output is reduced to cause bit errors, while in the .DELTA..theta./2-DSK signal, even if the phase difference .phi. takes on arbitrary values, the demodulation signal does not drop seriously owing to the compensation mechanism described above. As a result, the bit errors rate is extremely reduced.
Since the .DELTA..theta./2-DSK signal is constructed as described above, the bit error rate is remarkably improved, as compared with the BPSK signal. However, .DELTA..theta./2 could not be selected so that the best signal to noise ratio and complementary characteristics shown in FIG. 4 may be obtained, since the amount of .DELTA..theta./2 was related to the both amplitude coefficient cos(.DELTA..theta./2) and the complementary characteristic (the relation between output signals in the sections b and d and the section c).